The purpose is to determine the neurophysiological control of phonation and the pathophysiology of voice disorders such as spasmodic dysphonia. The normal timing and different motor control functions (respiration, effort closure, swallow, phonation and speech) were examined. For speech, the muscles were independently activated at different times and the activation pattern was more precisely controlled than during swallow or phonation. The intra-subject muscle relaxant effects of diazepam found on the laryngeal muscles in all normal subjects studied thus far have been independent from diazepam effects on muscle activation patterns for speech and swallow. Consistent decreases in the percent activation for speech and swallow occurred in some subjects and significant increases in others. These individual differences may relate to a subject's age. Patients with spasmodic dysphonia who have had unilateral recurrent laryngeal nerve resection are being studied neurophysiologically. Although all have vocal fold paralysis, close to normal activation levels have been found in the thyroarytenoid muscles on the resectioned side. Symptoms were reduced both by a temporary recurrent nerve block reducing muscle activation on the resectioned side and by botulinum toxin injection into the thyroarytenoid muscle on the same side, suggesting reinnervation following surgery. Nerve block and stimulation studies are being conducted in these patients to evaluate this possibility. Studies of the effects of acoustic reflexes, nerve stimulation and sensory nerve block on laryngeal muscle activation, have been initiated in patients with spasmodic dysphonia and normal volunteers.